(1) Field of the Invention
The present invention relates to a toner for developing a statically charged image and a process for the preparation thereof. More particularly, the present invention relates to a toner for developing a statically charged image, which has a good fixing property, a high offset resistance, a property of forming an image having a high quality and a good durability in combination, and a process for the preparation of this toner.
(2) Description of the Related Art
In commercial electrophotography, there is adopted an image-forming system in which a statically charged image is formed on a photosensitive material, the statically charged image is developed with a charged toner, the formed toner image is transferred onto a copy sheet or the like and the transferred toner image is thermally fixed. In view of the operation facility and safety, a method of passing a copying sheet carrying a toner image thereon through a heating roll is generally adopted for effecting the heat fixation. However, in this case, the problem of offsetting of the toner on the roll surface arises during heat fixation.
Many means for preventing this offsetting phenomenon have been proposed. For example, there is generally adopted a method in which an offset-preventing liquid is applied onto the roll or a release agent is incorporated into the toner.
Furthermore, many methods for preventing the offset by adjusting the molecular weight or viscoelastic (rheological) characteristics of the fixing resin in the toner have been proposed. For example, Japanese Unexamined Patent Publication No. 56-16144 teaches use of a fixing resin having one maximum value in a molecular weight range of 10.sup.3 to 8.times.10.sup.4 and one maximum value in a molecular weight range of 10.sup.5 to 2.times.10.sup.6 in the gel permeation chromatography (GPC), and Japanese Unexamined Patent Publication No. 59-214860 teaches that a thermoplastic resin in which the real part (storage modulus of elasticity) of the complex modulus of the elasticity is 5.times.10.sup.4 to 5.times.10.sup.6 Pa and the imaginary part (loss modulus of elasticity) is 5.times.10.sup.4 to 2.times.10.sup.6 Pa is used as the fixing resin.
According to the former proposal, it is taught that the low-molecular-weight component drops the lower limit of the fixing temperature and the high-molecular-weight component exerts the function of giving an offset resistance. However, if the amount of the low-molecular-weight component is too small, it is difficult to sufficiently drop the lower limit of the fixing temperature, and if the amount of the high-molecular-weight component is too large, the offset resistance is degraded. Accordingly, it is difficult to keep a good balance between the amounts of the two components, and satisfactory results cannot be obtained. The latter proposal is significant and interesting in that the storage modulus and loss modulus of elasticity of the resin are noted, but no strict correspondence relation is present between these factors and the actual fixing property or offset resistance.
These proposals relate to selection of a fixing resin to be used. However, in the case where the practical toner is evaluated, even if the resin reaches the selection standard, the fixing temperature range or the frequency of occurrence of the offset varies widely according to the dispersion state of the pigment or charge-controlling dye, and no satisfactory results can be obtained.